Although numerous studies have been reported on the possible role of an infectious agent(s) in the pathogenesis of rheumatoid arthritis (RA), the cause of this disease(s) remains elusive. Nonetheless, the possibility that a microbial agent may be involved will probably continue to challenge investigators until the etiology of RA is firmly established. A possible microbial etiology is based on findings that many microbial agents can cause naturally occurring or experimentally induced acute or chronic joint disease in animals and these infectious arthritides have clinical and pathologic features and autoimmune and immunologic abnormalities in common with rheumatoid arthritis. The present study was designed to carefully examine a relatively small number of synovial and spinal fluids from RA, non-rheumatoid (NRA) and schizophrenic patients for the presence of mycoplasmas, ureaplasmas, spiroplasmas and chlamydiae using a battery of media formulations and cell culture lines, each with special advantages and shown capable of isolating or detecting these fastidious agents. Synovial and spinal fluids and sera were also examined for antibody responses to ten Mycoplasma species, eight U. urealyticum serovars and to S. mirum. However, we failed to isolate mycoplasmas, ureaplasmas, spiroplasmas or chlamydiae from synovial or spinal fluids of ten rheumatoid (RA), six NRA or twenty schizophrenic patients. Whereas some of the 35 RA, 12 NRA and 20 schizophrenic patients had low serum antibody titers against some of the 18 mycoplasma species examined, only 4 RA patients had moderate titers to M. pneumoniae, 1 to U. urealyticum serovar SV, 4 to SVII and 13 to S. mirum SMCA. Seven RA synovial fluids had low antibody titers to S. mirum but the NRA fluids were negative. Our studies do not support the contention that these agents play a major role in the pathogenesis of these diseases.